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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 2 Aug 2019 11:24:33 +0100 Received: from d06av26.portsmouth.uk.ibm.com (d06av26.portsmouth.uk.ibm.com [9.149.105.62]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x72AOW3649938486 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 2 Aug 2019 10:24:33 GMT Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DE67DAE04D; Fri, 2 Aug 2019 10:24:32 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 92CD7AE045; Fri, 2 Aug 2019 10:24:32 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.152.224.71]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 2 Aug 2019 10:24:32 +0000 (GMT) To: David Hildenbrand , Igor Mammedov , qemu-devel@nongnu.org References: <20190729145229.4333-1-imammedo@redhat.com> <0e9cd550-c69b-3e97-4619-6746da4d4bd1@redhat.com> <63d203d2-25b5-44e6-ccee-1857396556f0@de.ibm.com> From: Christian Borntraeger Openpgp: preference=signencrypt Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 19080210-0008-0000-0000-00000303AE01 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19080210-0009-0000-0000-00002272B45A Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-08-02_05:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=786 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908020106 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [generic] X-Received-From: 148.163.156.1 Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH RFC 0/2] s390: stop abusing memory_region_allocate_system_memory() X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: pbonzini@redhat.com, qemu-s390x@nongnu.org, dgilbert@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 02.08.19 10:37, David Hildenbrand wrote: > On 02.08.19 10:29, Christian Borntraeger wrote: >> >> >> On 02.08.19 10:04, David Hildenbrand wrote: >>> On 29.07.19 16:52, Igor Mammedov wrote: >>>> While looking into unifying guest RAM allocation to use hostmem backends >>>> for initial RAM (especially when -mempath is used) and retiring >>>> memory_region_allocate_system_memory() API, leaving only single hostmem backend, >>>> I was inspecting how currently it is used by boards and it turns out several >>>> boards abuse it by calling the function several times (despite documented contract >>>> forbiding it). >>>> >>>> s390 is one of such boards where KVM limitation on memslot size got propagated >>>> to board design and memory_region_allocate_system_memory() was abused to satisfy >>>> KVM requirement for max RAM chunk where memory region alias would suffice. >>>> >>>> Unfortunately, memory_region_allocate_system_memory() usage created migration >>>> dependency where guest RAM is transferred in migration stream as several RAMBlocks >>>> if it's more than KVM_SLOT_MAX_BYTES. >>> >>> So if I understand it correctly, we only call >>> memory_region_allocate_system_memory() in case the guest initial memory >>> size exceeds KVM_SLOT_MAX_BYTES - ~8TB. >> >> We always call it. We just call it twice for > 8TB > > Yeah, that's what I meant. > >>> >>> Do we *really* care about keeping migration of systems running that most >>> probably nobody (except Christian ;) ) really uses? (especially not in >>> production). >>> >>> I am fine keeping migration running if it's easy, but introducing hacks >>> (reading below) for such obscure use cases - I don't know. >>> >>> @Christian: Please prove me wrong. :) >> >> For the time being we can block migration for guests > 8TB if that helps (it should not >> fail in a guest killing fashion), but we should >> 1. continue to be able to migrate guests < 8TB >> 2. continue to be >> >> On the other hand I find "and suddenly it fails if you go beyond this" really >> unpleasant. So it would be interesting to see the next round of patches to >> check how "hacky" those really are. > > I mean migration will work perfectly fine once we fixed it for new QEMU > versions. It's only the older QEMU versions to/from the > fixed one. I think that would be fine. We just have to make sure that migration really fails in a way that the system continues to run. > > Looking at the log I can see that this was introduced with v2.12.0. > > I would document this in the next release notes: "Migration of unusual > big VMs (>= 8TB) will not work from/to previous QEMU versions (up to > v2.12, before that starting such guests didn't even work)." Yes, sounds good.